750 point space marine army list. Any tips?

Hey all, I've had a space marine army for about 3 years, and hardly anyone to play with; but recently my friends got into the game, and have walloped me every time. I've played 6 games and lost them all. So today I set about building an army list to hopefully stand a bit more of a chance. The three armies that I play against are Orks, Space wolves, and Grey Knights. here is my army, let me know what you think, and how I can improve it.

grand total of 750 points. Tell me what you guys think. Is the army good? powerful? weak? need improvement? if so how and why and how can I fix it. I can use any pointers you have. Also if you have any tips on tactics I should use with this army those would be helpful as well. Thank you all.

1) Don't mix weapons in your dev squad. With combat squads you can split them up, which means 2 of each isn't that bad, but your current config isn't that hot. Las-cannons make more sense in tac squads, since they're cheaper, and you usually only need a single hit to do some good.

2) Need to get a second special weapon in the second tac squad. With that range of opponents, just about anything would be good.

3) You might want to swap out the sniper rifles for cc scouts. Sniper rifles are pretty useless against power armor.

I changed out the devs to all missile launchers, and put a meltagun in the second tactical squad. I used the extra 25 points left over to arm the first tactical squad's sergeant with a combi-flamer, and a power weapon because my main intent for them is to advance into CC. I also gave the scouts close combat weapons as you said but have a question about them because I have never armed scouts with close combat weapons. Does this mean that they gain an attack each( originally having one with the bolt pistol, and now a second with the combat blade)? With the combat scouts i'll intend to infiltrate, or if I deploy second, use the rhino for some cover as I advance.